During the assembly of semiconductor packages, semiconductor chips are often attached onto carriers, such as substrates or leadframes, for processing multiple semiconductor chips at the same time. After or during attachment, electrical connections are made between electrical pads on the chips to corresponding contacts or connection pads on the substrates or leadframes. This can be done by wire bonding, or the electrical pads can be directly attached onto the contacts on the substrates or leadframes. Thereafter, it is usually necessary to protect the chips and the electrical connections from the environment by encapsulating them in a molding compound, such as epoxy molding compound (“EMC”).
In a typical transfer molding process, the substrate or leadframe with the chips attached is placed into a molding system comprising two mold halves. One or more molding cavities are formed in one or both of the mold halves corresponding to the positions of the chips to be encapsulated. Molding compound is introduced into mold supply pots in the molding system, typically in pellet form. The mold supply pots are linked to the molding cavities through a system of runners and gates through which the molding compound is channeled before entering the cavities. A plunger is insertable into each pot under heat and pressure to crush the pellet and distribute molding compound under the pressure from the plunger through the system of runners and gates and into the molding cavities.
FIG. 1 is a plan view of a surface of a conventional mold chase 100. Only one mold half is shown, which cooperates with another mold half to clamp a carrier for molding. The mold chase 100 includes cavities 102 which are configured for molding semiconductor chips located on leadframes 104. The parts of a leadframe 104 that require molding are positioned onto corresponding cavities 102 of the mold chase 100 during onloading.
Molding compound is introduced to the mold chase 100 via pellets (not shown) that are crushed at the cull areas 106 to liquefy the molding compound. The liquefied molding compound travels from the cull areas 106 into runners 108 and are then introduced into the cavities 102.
During molding, it is preferable that the cavities 102 are in a substantially vacuum condition so that air gaps will not form in the cavities 102 during molding. Therefore, vacuum holes 110 are formed outside the leadframe 104 location and near to the periphery of the mold chase 100. The vacuum holes 110 are connected to a vacuum pump (not shown) to suck out air from the cavities 102.
Air vents 112 are formed adjacent to the cavities 102 so that paths are created for the air inside the cavities 102 to flow towards the vacuum holes 110. For comparison, the air vents 112 are only illustrated on one set of molding cavities 102 for molding a leadframe 104, but it should be appreciated that air vents 112 should be present with respect to all the cavities 102. The air vents 112 are in the form of depressions located in between clamping areas 114 of the mold chase 100 that are operative to clamp the leadframe 104 tightly onto another mold half during molding.
In order to maintain a vacuum pressure in the mold chase 100, the vacuum area has to be sealed off from the ambient atmosphere. To perform sealing, a seal ring 116, typically in the form of a gasket or O-ring, is used to surround the vacuum holes 110 near to the periphery of the mold chase 100. A relief area 118 is formed just inside the seal ring 116 adjacent to the vacuum holes 110 to create paths for air to travel between the cavities 102 and air vents 112 into the vacuum holes 110.
While the above design is suitable for most molding purposes, it is not ideal as the seal ring 116 is vulnerable to wear failure after prolonged use and exposure to heat. Typically, it lasts only for a few months. Also, it is prone to damage, for example when it accidentally comes into contact with a sharp foreign material, especially when the mold is being closed to clamp the carrier to be molded.
There is a further risk of mold bleed occurring during molding such that the molding compound may then come into contact with the seal ring 116. This will also cause damage to the sealing ring 116. Meanwhile, since the seal ring 116 is exposed, there is always a risk of other accidental damage occurring due to human error.